The invention relates to the field of electronic reproduction technology and relates to a method for the thermal stabilization of a laser diode in the point by point and line by line exposure of recording material by means of a light beam in an electronic exposing device, also referred to as an exposer, recorder, or image setter. A recorder is used, in particular, to expose printing forms, that is to say films or printing plates, which contain all the text, graphics and image elements of a page to be printed. In the case of multicolor printing, a separate printing form is exposed for each printing color.
In a recorder, a light beam that is modulated by an image signal is guided point by point and line by line over the recording material to be exposed. The recording material is thereby fixed on a holder which moves relative to the light beam. In the case of an internal drum appliance, the recording medium is fixed onto a holder shaped like a segment of a cylinder or an exposure trough, and the light beam is guided over the recording material point by point and line by line by means of a rotating light beam deflection device. However, the recorder can also be constructed as an external drum apparatus or flat bed apparatus.
The light source often used to produce the light beam is a laser diode, which is modulated with the image signal. In conventional printing technology, different densities of a printing ink are produced by means of raster points of varying size, that is to say ink is either printed or not at each point on the printed page. The image signal therefore has only two signal values, namely, ink or no-ink or exposure or non-exposure. Accordingly, the laser diode is modulated during the exposure in such a way that it emits light for the image points to be exposed and does not emit light for the image points that are not to be exposed. Driving the laser diode by means of the image signal is carried out in the prior art in such a way that it is activated and modulated line by line within exposure time periods, during which the light beam sweeps over the lines to be exposed on the recording material, and is deactivated within return time periods, wherein the light beam is guided to the next line to be exposed.
A known problem in the case of exposure with a laser diode is that the light power emitted and also the wavelength change with the temperature of the laser diode. In this case, different time constants apply to the temperature change, very short time constants when the temperature of the PN junction in the laser diode changes very quickly as a result of the modulation current being switched on and off, medium time constants when the temperature of fittings of the laser diodes changes as a result of a varying medium current loading, and a long time constant if, for example, the ambient temperature changes. As a rule, the short, medium, and long time constants cannot be delimited clearly, instead there is a sliding transition between the temperature dependencies with different time constants. In the individual case, this depends to a great extent on the construction of the laser diode, that is to say on the mechanical-thermal attachment of the laser chip to a carrier plate and the thermal coupling via further fittings as far as the housing. In addition, the temperature time constants are influenced by the design configuration of the installation of the laser diode in the recorder, the dissipation of heat via the components of the recorder and the ventilation.
In order to expose printing forms, a constant light power is imperative, since fluctuations in the light power manifest themselves in undesirable stripes, patterns, and the like in the finally exposed printing form. Furthermore, a wavelength which changes as a result of temperature fluctuations can also bring about similar disruptive patterns, depending on the relationship between the exposure sensitivity of the recording material and the wavelength of the light.
In order to compensate for very short-term power fluctuations, which occur as a result of the temperature change of the PN junction of the laser diode during a single modulation pulse, a method is known wherein the drop in power caused by the increase in temperature or the increase in power caused by a reduction in temperature is compensated for by means of an additional correction current. In this case, the time variation of the power change is compensated for by an appropriate opposite variation in the correction current, the time variation being simulated by a resistance-capacitance network in the current generator for the correction current. Such a method is described in U.S. Pat. No. 5,309,458.
In order to compensate for power fluctuations with medium and long time constants, according to the prior art, control of the light power is used, part of the light beam being coupled out of the exposure beam path and deflected onto a light sensor. The light power measured by the light sensor is input as a measured variable into a control loop, which controls the power of the laser diode to a predefined value. To this end, for example during a control phase before each modulation phase, the light power at a predefined laser diode current is measured and the laser power is controlled. Such a method is described in U.S. Pat. No. 5,495,463.
The control method is primarily suitable to compensate for relatively slow temperature changes from modulation phase to modulation phase. In the case of a recorder for printing forms, a line to be exposed corresponds to one modulation phase. The control method is able to intervene and compensate for deviations only before each line to be exposed. In addition, the control method is complicated to implement, particularly if the intention is also to control out power fluctuations from line to line, which are brought about by the number, which depends on the image content, and also by the distribution, which varies within the lines, of the image points to be exposed. The correction current method is suitable only for very short power fluctuations within the time range of one or a few image points, and cannot therefore compensate for the power fluctuations with a medium time constant.
It is accordingly an object of the invention to provide a method of thermally stabilizing a laser diode in a recorder, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which improves a method of exposing recording material with the laser diode in such a way that thermal stabilization of the laser diode power in the range of medium time constants is achieved in a simple way.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for thermal stabilization of a laser diode in a recorder, which comprises:
modulating a laser diode by superimposing a basic current and a modulation current and generating a modulated light beam with a laser diode;
exposing recording material point by point and line by line with the modulated light beam, during exposure periods, wherein a respective line is exposed, and return periods, wherein the light beam is guided to a start of a respectively next line; and
thereby modulating the laser diode within the return period.
In accordance with an added feature of the invention, the laser diode is modulated with the basic current during a stabilization period.
In accordance with an additional feature of the invention, the sum of the modulation times with the basic current during the exposure period and during the stabilization period is constant.
In accordance with another feature of the invention, the sum of the modulation times with the basic current during the exposure period and during the stabilization period corresponds to the maximum line length that can be exposed.
In accordance with a further feature of the invention, before the first line is exposed, the laser diode is modulated with the basic current during the stabilization period.
In accordance with again an added feature of the invention, in a start/stop operating mode of the recorder the laser diode is modulated with the basic current during the stabilization period during a stop phase and a restarting phase.
In accordance with again an additional feature of the invention, during the return period, the laser diode is additionally modulated with a modulation current.
In accordance with again another feature of the invention, the modulation current is derived from the inverted image signal from the preceding or the following line.
In accordance with again a further feature of the invention, the modulation current is lengthened to the maximum possible line length by an appended signal component.
In accordance with yet a further feature of the invention, the appended signal component is shortened by a proportion of the time which corresponds to the emitted light power from the laser diode during the exposure period and the stabilization period.
In accordance with a concomitant feature of the invention, the recorder is an internal drum recorder.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for the thermal stabilization of a laser diode in a recorder, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.